1. Field of the Invention
The present invention relates to a method of forming a fine wiring pattern to be used in array substrates for flat-panel display devices.
2. Description of the Related Art
A recent movement in the field of electronic display is the replacement of CRT displays by flat-panel displays which are under active development. Among others, liquid crystal display devices are attracting attention because of their advantages of being thin in size, light in weight, and low in power consumption.
They are illustrated below with a liquid crystal display of transmittive, active matrix type having switching elements arranged for individual pixels. This display is composed of an array substrate and a counter substrate, with a liquid crystal layer interposed between them via alignment layers. The array substrate is a transparent insulating substrate of glass, quartz, etc. on which signal lines and scanning lines are arranged in a lattice pattern. Each of their intersections is connected to a thin film transistor (TFT for short hereinafter) which employs semiconductor thin film of amorphous silicon (abbreviated as a-Si:H hereinafter). The TFT has its gate and drain electrodes electrically connected to the scanning and signal lines, respectively, and has its source electrode electrically connected to the transparent electrically conductive material (such as ITO: Indium-Tin-Oxide) constituting the pixel electrode.
The liquid crystal display of active matrix type as mentioned above is widely used for notebook-size personal computers, car navigation systems, and very small TV sets.
For the flat-panel displays to gain wider acceptance in the market, they need higher resolution and larger size.
To achieve the object of improving resolution, it is necessary to reduce the pixel pitch (distance between the centers of adjacent pixels). To maintain a large aperture ratio, it is necessary to form a fine pattern with narrow lines and small line spaces. In this specification, xe2x80x9cpixelxe2x80x9d denotes a dot of each color (e.g., red, green, and blue) for the color display. In addition, in the case where the pixel pitch differs depending on the vertical and horizontal directions of the display panel, xe2x80x9cpixel pitchxe2x80x9d denotes the smaller one.
For a 20-inch (diagonal) monitor to meet the UXGA specification (1600xc3x973xc3x971200 pixels), it is necessary that the pixel pitch be about 0.25 mm. In this case, the smallest wiring width and the smallest wiring space (space between adjacent wiring patterns) should be less than about 30 xcexcm and 100 xcexcm, respectively, so that a satisfactory aperture ratio is secured for efficient light utilization. Also, for a 20.8-inch (diagonal) monitor to meet the QUXGA specification (3200xc3x973xc3x972400 pixels), it is necessary that the pixel pitch be about 0.132 mm and the wiring width be less than 15 xcexcm.
On the other hand, in order to increase the size of a display panel while reducing the wiring width, it is necessary to make the wiring from a low-resistance metal which does not deteriorate the waveform being transmitted. Aluminum is known to meet this requirement.
Unfortunately, conventional technologies present difficulties with deposition by sputtering of a metal layer with uniform thickness because sputtering causes metal particles to stick to the metal layer, resulting in minute defects. Such particulate foreign matter (splash) on the metal layer leads to defective wiring after patterning if it is larger than 5 xcexcm in diameter and the wiring width is smaller than 30 xcexcm or the wiring space is smaller than 60 xcexcm. How splash is formed is schematically shown in FIG. 18.
The occurrence of splash by sputtering is more frequent in the case of aluminum than in the case of molybdenum which has been commonly used for liquid crystal display devices. Splash of aluminum remarkably increases if aluminum is incorporated with neodymium to protect the aluminum layer from hillocks (bulging due to heat process), as proposed in JP-A-07 045555 (1995) (Japanese Patent Laid-open No. 45555/1995).
One way to prevent hillocks when an aluminum layer is formed by sputtering is to carry out sputtering in an argon atmosphere at 3-50 mTorr (0.4-6.7 Pa), preferably about 10 mTorr (1.3 Pa), according to JP-A-08 037186 (1996) (Japanese Patent Laid-open No. 37186/1996). It is claimed that sputtering in this way gives rise to aluminum alloy containing 0.1-6.5 atom % argon because the aluminum layer takes up argon. However, it was found that it is impossible to prevent splash simply by carrying out sputtering in an argon atmosphere at a specific pressure.
The present invention was completed to address the above-mentioned problem. Accordingly, it is an object of the present invention to provide a method of forming a fine wiring pattern by sputtering and patterning without causing defects due to splash.
The gist of the present invention resides in a method of forming a fine wiring pattern which includes a first step of depositing by sputtering on an insulating substrate a thin film of aluminum or alloy containing more than 70 atom % aluminum and a second step of patterning said thin film, thereby forming a fine wiring pattern, wherein said sputtering is carried out under the condition that the potential difference between the anode and the cathode is lower than 570V. Sputtering in this way does not cause wiring defects due to splash.
According to one embodiment of the invention, the fine wiring pattern has a wiring width smaller than 30 xcexcm or a wiring space smaller than 60 xcexcm.
According to another embodiment of the invention, the fine wring pattern is formed from an aluminum alloy containing neodymium (Nd), yttrium (Y), and/or gadolinium (Gd) in a total amount of 0.3-5.0 atom %. Sputtering in this way prevents the occurrence of hillocks and wiring defects due to splash.
According to further another embodiment of the invention, thin film is deposited on a substrate placed in a reaction chamber by DC sputtering. This process comprises a step of detecting a decrease in the cathode-anode voltage, thereby detecting the occurrence of arc discharge, a step of cutting off power supply to apply voltage across the cathode and anode within 1 xcexcs after the occurrence of arc discharge, and a step of resuming power supply and sputtering within 5-15 xcexcs after said power cut-off.
The advantage of this embodiment is that it is possible to prevent the occurrence of splash due to arc discharge, thereby improving the yields of array substrates, and it is also possible to greatly reduce downtime that is required for operation to resume after the maintenance of the sputtering chamber has been completed.
According to still further another embodiment of the invention, thin film is deposited on substrates by DC sputtering in such a way that a prescribed sputter voltage is applied across the target electrode and the substrate and a sputter magnet is moved back and forth parallel to the target electrode. This process is characterized in that the sputter magnet is moved vertically with respect to the target electrode, thereby adjusting the distance between the sputter magnet and the target electrode, so as to decrease fluctuation in the sputter voltage when it is detected that the sputter voltage has changed from its reference voltage.
The advantage of this embodiment is that it is possible to easily keep the sputter voltage constant and hence it is possible to eliminate wiring defects due to splash and to form thin film with uniform quality. In the case of DC sputtering apparatus provided with a mechanism to move the sputter magnet back and forth to form thin film on a large substrate, it is possible to prevent the sputter voltage from increasing due to the action of the shield screening the periphery of the target. This leads to thin film of uniform quality.
The DC sputtering apparatus used in this embodiment consists of a treating chamber for substrate treatment, a target electrode arranged in said treating chamber, a sputter magnet so supported as to move back and forth parallel to said target electrode, a voltage detecting means to detect sputter voltage, a mechanism to move said sputter magnet vertically with respect to said target electrode, thereby adjusting the distance between said sputter magnet and said target electrode, and a mechanism to reduce fluctuation in said sputter voltage by adjusting the distance between said sputter magnet and said target electrode.
The DC sputtering apparatus used in another embodiment consists of a treating chamber for substrate treatment, a target electrode arranged in said treating chamber, shelf-like shields which are arranged near the periphery of said target electrode so as to prevent plasma from impinging on said target electrode, a sputter magnet so supported as to move back and forth parallel to said target electrode, a mechanism to move said sputter magnet vertically with respect to said target electrode, thereby adjusting the distance between said sputter magnet and said target electrode, and a mechanism to reduce the distance between said sputter magnet and said target electrode when said sputter magnet is positioned in the region where said target electrode is screened by said shelf-like shield.